It has been heretofore widely known to improve corrosion resistance of a steel material by applying Zn coating to the steel material surface, and a steel material subjected to Zn coating is being mass-produced at present. However, in many applications, corrosion resistance imparted only by Zn coating may be insufficient. Therefore, as a steel material is more enhanced in the corrosion resistance than by Zn, a hot-dip Zn—Al alloy-coated steel sheet (Galvalume Steel Sheet (registered trademark)) is being used. For example, the hot-dip Zn—Al alloy coating disclosed in Patent Document 1 is performed by applying an alloy coating composed of Al in a concentration of 25 to 75 mass % and Si in a concentration of 0.5% or more of the Al content, with the balance being substantially Zn, where a hot-dip Zn—Al alloy coating layer not only being practically excellent in corrosion resistance but also having good adherence to a steel material and good-looking appearance is obtained.
As another method for enhancing the corrosion resistance of Zn, Zn—Cr-based alloy coating of adding Cr to the coating layer has been proposed. The Zn—Cr alloy coating disclosed in Patent Document 2 is applied, as a coating layer, a Zn—Cr-based alloy electrocoating layer composed of Cr in a concentration of more than 5% and 40% or less, with the balance being Zn, where excellent corrosion resistance is obtained compared with a steel sheet subjected to conventional Zn-based coating.
Patent Document 3 discloses a technique where as a result of adding various alloy elements to a coating based on Zn-55% Al that is the coating composition of Galvalume Steel Sheet and studying the addable amount thereof or the corrosion resistance-enhancing effect by the addition, a coating containing Al: 25 to 75 mass % can contain Cr in a concentration of about 5 mass % and the corrosion resistance can be remarkably enhanced by containing Cr. This is a technique of increasing the corrosion resistance by forming a Cr-concentrated layer at the interface.
Also in Patent Document 4, various alloy elements are added to a coating based on Zn-55% Al that is the coating composition of Galvalume Steel Sheet, and the addable amount thereof or the corrosion resistance-enhancing effect by the addition is studied, where in particular, a technique of enhancing the bending processability by optimizing the spangle size of coating is disclosed.
Furthermore, Patent Document 5 also discloses a technique of enhancing the processability by controlling the particle size of an interfacial alloy layer formed by coating with the Galvalume composition.